Quantum walk comb in a dual waveguide quantum cascade laser

Ring quantum cascade lasers (QCLs) have recently demonstrated very stable active comb formation in the form of a quantum walk comb which takes advantage of very short gain recovery mechanisms, inducing liquid properties to the light. Due to the ring geometry, applications that require high output power remain limited. Here we show a 2 orders of magnitude light outcoupling improvement, up to a maximum of 120 mW at a laser heatsink temperature of 253 K, while still reaching a clean quantum walk comb state. These results are achieved by homogeneously integrating a racetrack QCL with a passive waveguide, which lies on top of the active region plane and therefore can be designed fully independently from the laser geometry. We also prove that we can change the light dispersion by tuning the parameters of the passive waveguide, with a direct impact on the behavior of the generated comb. Time-resolved spectral measurements in such devices demonstrate the ballistic expansion characteristic of quantum walk combs up to a bandwidth of 30 cm^-1.